Powdered zinc sulfide electroluminescent phosphors and corresponding ACPEL applications have been known for a long time. The electroluminescence-capable ZnS phosphors are usually doped with copper (Cu) and/or manganese (Mn), wherein, in addition, other monovalent or trivalent ions (for example, the ions of the elements Cl, Br, I and/or Al), which function as coactivators, can be incorporated in the base lattice. The grain size of the synthesized materials is generally in the μm range. The application usually occurs in the form of so-called electroluminescent films, in which the phosphor particles, in the sense of a capacitor arrangement, are arranged between two electrodes and insulating layers and, upon application of an alternating electrical field of, usually, 110 V and 400 Hz, they emit light in the blue, green or orange region of the visible spectrum. Application examples for such electroluminescent films or lamps are display lights, background lights, lighting and marking elements as used in aircraft and motor vehicles, in buildings or for the production of advertising installations.
The zinc sulfide phosphor particles used for producing known electroluminescent films are usually provided with thin water vapor barrier layers, in order to increase the useful life of the films. This coating, which is also referred to as microencapsulation, can occur, for example, by means of processes such as chemical vapor deposition. The layers that completely encase the individual phosphor particles can consist of SiO2, TiO2, Al2O3 (cf. U.S. Pat. Nos. 5,156,885 A, 5,220,243 A), of oxynitrides of the elements Al, B, Si, Ti (cf. WO 00/022064 A1) or of AlN (cf. WO 98/24254 A1)
It is also known to use microencapsulated or non-encapsulated ZnS-EL pigments in powder form as security features in security documents and documents of value such as, for example, in bank notes, travel passports, identity cards, driver's licenses, etc. (cf. EP 0 964 791 B1). Here, the zinc sulfide electroluminescent phosphors used in these cases for the purpose of protection against counterfeiting (cf. EP 1 151 057 B1) are usually arranged by means of conventional printing technologies (for example, gravure printing, offset printing or screen printing processes) on or in the matrix of the respective security documents, which can consist of paper, plastics, laminated plastics or also of other suitable materials, without in the process seeking to produce or producing a conventional capacitor structure. For authenticity verification, the electroluminescent phosphors thus applied are preferably excited in a contact-free manner with electrical fields, wherein, due to the specific and unconventional arrangement of the electroluminescent pigments in the matrix of the security document, relatively high-frequency high-voltage fields are generally needed in order to ensure a reliable stationary and advantageously also high-speed detection of the resulting luminescence signals (cf. EP 1 059 619 B1, EP 1 149 364 B1 and DE 10 2008 047 636 A1).
An increase of the local field strength of the alternating electrical fields applied for the excitation of the electroluminescence, acting on the electroluminescent phosphors, can also be achieved by arranging, in addition to the EL pigment and in the immediate vicinity thereof, so-called field suppression elements in the corresponding security markings of the security documents and/or documents of value (cf. EP 1 631 461 B1, EP 1 748 903 B1). The field suppression elements are insulated, electrically conductive pigments with high dielectric constants, wherein either metal particles consisting of iron (Fe), copper (Cu), aluminum (Al) and/or silver (Ag), or certain transparent, optically variable multi-layer effect pigments are used.
Electroluminescent security features of the described type have a very high security level and are generally considered to belong to the category of the so-called level 3 features. The authenticity confirmation of corresponding security documents requires special knowledge of the mode of operation of the feature and is associated with very high cost and very strict requirements for the detection technique used.
If, as described in EP 1 748 903 B1, for the purpose of increasing the effective local strength of the excitatory electrical field and thus the signal strength of the resulting electroluminescence of the feature, in addition to the EL phosphors, so-called field suppression elements in the form of electrically conductive, optically variable effect pigments are used, then the feature can have, in addition to the level 3 characteristic, also a corresponding level 1 characteristic. The optical effect of these pigments, which consists of a color change that can be perceived by the observer under different illumination angles and viewing angles, can be used as an additional feature in the authenticity verification.
ZnS phosphors doped with different activator ions can also be excited to luminescence by means of electromagnetic radiation (for example, with UV rays or X-rays), or by means of electron beams. Corresponding applications as phosphors for conventional color picture tubes or as phosphorescent pigments have a long tradition. However, as a general rule, the zinc sulfide phosphors optimized for efficient photoluminescence, cathode luminescence or X-ray luminescence have no electroluminescence or at least no usable electroluminescence. The same applies conversely: the EL pigments used in conventional technical applications as well as those used in security documents and documents of value can be excited to luminescence by electromagnetic radiation and generally cannot or can only very weakly be excited to luminescence by electron beams, which is explained by the fundamentally different luminescence mechanisms of the different luminescence types as well as by the compositions of the EL pigments, which are selected especially for achieving high electroluminescence yields, and by the special production technologies (cf. Shionoya, S.; Yen, W. M.: “Phosphor Handbook,” CRC Press, 1999, pages 581-621).
The production of zinc sulfide electroluminescent phosphors occurs in principle on the basis of a multiple-step process of solid state chemistry. For this purpose, the following steps to be carried out one after the other are known from the state of the art, including from EP 1 151 057 B 1:
1. Intensive mixing of the starting substances in powder form
2. High-temperature annealing process in the temperature range between 900° C. and 1.300° C.
3. Washing of the annealed material with H2O and/or optionally with dilute mineral acids, wet grinding of the annealed material particles
4. Secondary doping of the intermediate product by renewed addition of predetermined quantities of CuSO4 
5. Thermal treatment of the dried material at temperatures between 500° C. and 900° C.
6. Washing of the product obtained after the tempering, with H2O, mineral acids such as HCl or HNO3, or in the presence of KCN, for the removal of Cu2S deposited on the surface
7. Renewed tempering of the dried phosphor powder at 300° C. to 500° C.
The high-temperature process, which is crucial for the formation of the activated and co-doped phosphor base lattice is here carried out regularly in a reducing atmosphere, i.e., this means that the annealing occurs in the presence of a gas mixture consisting of nitrogen and hydrogen, wherein the hydrogen content typically can be as much as 10%.
In the case of the use of highly innovative security features with level 3 status in security documents such as bank notes, travel passports, identity cards, driver's licenses, for example, the disadvantage is that it is then always impossible to demonstrate with certainty the presence of the respective security feature, if the laboratory equipment or the sensor system required for this purpose is not present, or if using it would entail an unjustifiably high expense. This can also apply to a particularly high degree to electroluminescent security features. Additional reasons not to use such security features include, for example, that environmental regulations or safety regulations do not allow carrying out the excitation of the EL pigments with high-frequency high-voltage alternating electrical fields, which is required for authenticity verification. Restrictions of the type described can apply, for example, to the verification of electroluminescent features in bank notes, if the authenticity confirmation is to be carried out in decentralized sorting installations or, on the other hand, in automated bank tellers for individual cash withdrawal or for cash payments. In such cases, it would be highly useful if the electroluminescent feature contained additional features independent of the level 3 characteristic thereof, which could also be used for the authenticity verification, without revealing the operating principle of the level 3 security feature. Preferably, in this alternative process (level 2 status), the authenticity confirmation of the feature should also be possible using detection means that are comparatively easy to handle.